Ask Americans to describe themselves, and chances are you'll get adjectives like "energetic," "friendly" or "hard-working."

In Japan, the responses would likely be much different. "Dependent on others" and "considerate" might pop up, studies have found.

Psychologists have known for a long time that people in East Asia think differently, on average, than do those in the U.S. and Europe. Easterners indeed tend to be more cooperative and intuitive, while Westerners lean toward individualism and analytical thinking.

Now psychologists have evidence that our ancestors planted some of these cultural differences hundreds of years ago when they chose which grains to sow.

"We call it the rice theory," says Thomas Talhelm, a graduate student at the University of Virginia who led the study. "Rice is a really special kind of farming."